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Journal of Magnetic Resonance138,277–280 (1999)Article ID jmre.1999.1729, available online at http://www.idealibrary.com on

A Low Frequency 1H-NMR External Unit for the Analysisof Large Foodstuff Samples

Francesco Capozzi,* Mauro A. Cremonini,† Claudio Luchinat,‡ Giuseppe Placucci,§,1 and Carlo Vignali¶

*Department of Chemistry, University of Calabria, 87036 Arcavacata di Rende, Cosenza, Italy;†Food Science and Technology Laboratory, UniversityBologna, Via Ravennate 1020, 47023 Cesena, Italy;‡Department of Soil Science and Plant Nutrition, University of Florence, Piazzale delle Cascin

50144 Florence, Italy;§Department of Organic Chemistry “A. Mangini,” University of Bologna, Viale Risorgimento 4, 40136 Bologna, Italy;and ¶Interfaculty Measure Center, University of Parma, Viale delle Scienze 23/A, 43100 Parma, Italy

Received December 7, 1998

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An inexpensive external unit that allows the use of a commercialigh-resolution NMR spectrometer as a very low frequency instru-ent is described. The external unit is phase coherent, the pulse

iming being given by the parent spectrometer. With the exceptionf the probe, the external unit does not contain any tuned ele-ents. This permits easy change of frequency in the range 100

Hz–1 MHz. The external unit may be appropriately employed inood science where, in several cases, low frequency is desirable. Anpplication to hen shell eggs at the frequency of 700 kHz isescribed. © 1999 Academic Press

Key Words: low field 1H-NMR; foodstuff quality; shell eggs;elaxation times; external unit.

INTRODUCTION

Relevant information about foodstuff quality can beained through the measurement of the longitudinal and terse relaxation times of the bulk water in the sample1).ften, such measurements are more sensitive to quality pters when performed at low field. The general reason li

he fact that water interacts with immobilized or slow-rotaissue components. This interaction changes its spectral dn such a way that large relaxation can only be observeelatively low field.

Low-field NMR spectrometers are far less expensiveheir high-field counterparts. This fact, however, doesustify the purchase of a commercial low-field equipmenthe sole purpose of testing the feasibility of a new applicaoreover, not every variety of low-frequency NMR spectroter is commercially available. Fortunately, old NMR elecagnets are still available in many laboratories. A devic

his type, together with a common high-resolution spectroer, a homemade external unit, and a suitable probe, cased to set up a versatile low-frequency apparatus at verost.In this paper we describe the assembly of the exteodules needed to permit the use of a Bruker CXP-200 s

1 To whom correspondence should be addressed.

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rometer as a very low field NMR machine working in02–103 kHz frequency range. For the sake of clarity, weefer to the setting-up of a 700 kHz prototype that we buiarry out a novel study on the aging process of shell egtudied by low-field NMR (2). Very few changes will bequired to use the equipment at another frequency in theange. The choice of specific commercial componentsictated by their availability and low price, and may thereot always represent theoptimumfrom a performance stanoint.

EXTERNAL UNIT ARRANGEMENT

The block diagram of the whole external unit working atHz is shown in Fig. 1. Note that no 700 kHz signal is tarom the CXP console. In fact, the CXP pulse modulannot handle frequencies lower than 2 MHz. Consequehe required frequency is obtained by mixing the CXP lower transmitter output, set at 9.3 MHz, with the local os

ator provided by the CXP 10 MHz master oscillator signauch a way as to make the 700 kHz frequency available aixer output. The latter signal is amplified by an untu

ascade amplifier and delivered to a homemade probe thpassive diode switch. In this way, any phase cycle issueCXP pulse program can be safely transferred to the 700

hannel.Similarly, the 700 kHz NMR signal from the probe

reamplified and back converted to 9.3 MHz by the samHz reference signal, before being applied to the CXP

nput. Phase coherence is achieved because the same 1eference is used for both the TX and RX circuits.

xternal Modules

Downconverter and RF amplifier.The downconverter anF amplifier circuit are shown in Fig. 2. The TX mixer is

ow-level and low-cost commercial double-balanced typerst transistor low-level stage operates in class A, whileext MJ481 buffer amplifier and the MJ481 output ampl

1090-7807/99 $30.00Copyright © 1999 by Academic Press

All rights of reproduction in any form reserved.

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278 CAPOZZI ET AL.

oth operate in class C. By using class C the amplifieutomatically turned off when the unit switches to receivode, thus making gating circuits unnecessary. The relat

ow frequency transistors used prevent the unwantedrequencies present at the mixer output from being ampliowever, a high-pass filter (f c 5 100 kHz) is also necessat the TX output to remove very low frequency noise androper impedance matching. No tuned circuits are used implifier chain, so the amplifier isflat from 100 kHz to 1 MHzhe amplifier provides 20 W RF output power.

TX/RX switch, receiver preamplifier, and mixer.The genral outline of the receiving part of the low-frequency NMxternal unit is shown in Fig. 3. The NMR signal coming fr

he probe reaches the preamplifier input through a TXwitch. Diodes are employed on the transmitter and recrms for passive switching, i.e., switching is provided byF excitation itself (3, 4). A single stage FET preamplifiandles the 1 V overload appearing at the TX/RX switutput and applies a 10 dB gain to overcome the conve

oss of the following upconverter. The latter is a117 dBmouble balanced mixer whose local oscillator port is drive

FIG. 1. Block diagram of the external unit. Because of the particularonsole as an heteronuclear probe tuned at 9.3 MHz. Here TX5 transmitreamplifier.

FIG. 2. Outline of the TX downconverter and power amplifier. Theomponents (f 5 100 kHz) areC 5 C 5 15 nF,L 5 60 mH.

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he 10 MHz frequency available from the CXP master oator, thus providing a 9.3 MHz upconverted NMR signal. Tocal oscillator frequency is not gated, so the output isected to a residual 10 MHz signal that is filtered out by a vigh Q crystal notch filter. A 30 pF variable capacitor permne notch tuning over a few kilohertz range around the nnal 10 MHz series resonance of the quartz crystal. The ss coupled to the source follower BF245A to match thenput impedance of the following 50-ohm line that feedsMR signal to the CXP-200 standard preamplifier.

Probe. The prototype 700 kHz probe (Fig. 3) has built large enough to contain a shell egg. The sample coilL 1)

s a simple solenoid wound around a common laboraeaker and connected to a capacitor, to form a series resircuit with aQ of about 40. This relatively high value imposquite long pulse rise and fall time, of the order of 30ms.

lthough the total dead time is even longer (about 200ms), thiss of no concern when samples with a sufficient longT1 andT2

0.01–1 s) aredealt with. With the present probe configuratnd the 20 W RF transmitter described above, a 60ms 90egree pulse and a 100ms 180 degree pulse were obtained.

ctronic configuration chosen, the 700 kHz NMR external unit appears tRX5 receiver, PA5 power amplifier, LO5 local oscillator, pre-amp5

L-1X mixer is a Mini Circuits double balanced ring type. The high-pa

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279NMR EXTERNAL UNIT FOR FOODSTUFF ANALYSIS

1 homogeneity has been measured by directly mappinF intensity within the probe and found to be within 95%t least 90% of the volume, which is where the greatest pashell egg is actually placed.The probe, together with the TX/RX switch, was contai

n an aluminum box and inserted between the poles of aarian 12-inch electromagnet modified to a 74 mm gap.lectromagnet was driven to 0.0164 T by a 1 A current proided by a voltage supply set at about 1 V.

APPLICATION

MR of Shell Eggs at 700 kHz

The 700 kHz frequency was chosen because prelimeld-cycling relaxometry experiments indicated that theess of egg aging is best mirrored into the albumen relaximes at frequencies lower than;1 MHz (2).

In the absence of any possibility of field shimming, aecause of the large volume of the sample, the linewidth oater protons of a shell egg is rather large (about 500–z). This turns out to be not particularly annoying when b1 (inversion recovery) (5) andT2 (spin-echo) (6) experimentre run. Besides, any unwanted magnetization due to the

FIG. 3. Circuit diagram of the probe, TX/RX switch, and RX upconvround a 50 mm diameter pyrex beaker (about 67mH). The circuit is tunedF variable capacitor, both working at 1 kV voltage. The series LC circuionsisting of 16 turns of self-supported 2 mm enameled copper wire. TXX upconverter: The SRA-1H is a Mini Circuits double balanced mixer. X

grade F19 core): Primary has 6 turns and secondary has 30 turns of 0

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1 inhomogeneity can be successfully eliminated with simhase cycles.As an example, the longitudinal magnetization recovery

he transverse magnetization decay curves of a shell eggre shown in Fig. 4. They have been obtained by plotting

ntensities of the Fourier transform of either the FIDs, innversion recovery, or the half-echo height in a spin-experiment. It is apparent that even at short times the exental points are well behaved, theS/N ratio being quite hig

better than 15 with a single scan). The clear at leas7)iexponential decay of both curves is actually the main suf our future research: We have evidence that changeselaxation curves with time after laying could be used to ihe freshness of an egg (2). Further details are beyond tcope of this article and will be presented in a subseqaper.

CONCLUSIONS

The inexpensive external unit presented in this papereen used with a Bruker CXP-200 NMR spectrometeetermineT1 andT2 relaxation curves of a large food sama shell egg) at 700 kHz. Although our experience is limitehe CXP console, we believe that the same approach c

r.robe:TheL 1 coil is made of 45 turns of 1 mm enameled copper wire wo00 kHz by a 750 pF nonmagnetic high-Q capacitor and is finely tuned by amatched to 50 ohm by inductorL 2, a 20 mm diameter solenoid, about 45 mm lo

switch: diodes are 1N4448 type.C1 5 C2 5 3.3 nF,L 3 5 15 mH (at 700 KHz).L is a surplus 10 MHz series-resonance quartz crystal.T1 is a toroidal transforme

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280 CAPOZZI ET AL.

ollowed with any other NMR console, provided a refereignal and a low-power transmitted output are available toser.The low resonance frequency allowed by the unit permit

asy construction of a probe capable of containing largeles, thus giving the possibility of performing nondestruc

FIG. 4. Longitudinal magnetization recovery and transverse magneion decay of the water proton signal of a shell egg at 700 kHz. Note thaturves cannot be fit with a single exponential function (dashed). A modeln the sum of two exponential functions, each having different weightsamping factors, is much more appropriate (solid), though not necessarest one (7). The relaxation time constants with the larger weight iniexponential fit (more than 80%) are indicated.

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nalyses of eggs. This feature, together with the fact thats no risk of microbial sample contamination during the asis, makes the approach suitable for on-line applicationsEven lower frequencies can be reached in a similar wa

nly modifying the probe and theC1, L 3, C2 network in theX/RX unit (4), the only limitations being the dimension of trobe and, of course, the lower sensitivity.

ACKNOWLEDGMENTS

G.P. and M.A.C. thank MURST (national project: stereoselection in orgynthesis) Rome and the University of Bologna (funds for selected resopics) for financial support. C.L. thanks MURST (ex 40%) and Ente Casisparmio di Firenze for partial support.

REFERENCES

. D. N. Rutledge, Low resolution pulse nuclear magnetic resonance inthe agro-food industry, J. Chim. Phys. 89, 273–285 (1992).

. F. Capozzi, M. A. Cremonini, A. Franchini, C. Luchinat, G. Placucci,and C. Vignali Abstract Book of the Fourth International Conferenceon Applications of Magnetic Resonance to Food Science, Septem-ber 1998, Norwich, UK.

. D. D. Traficante, Introduction to transmission lines. Basic principlesand applications of quarter-wavelength cables and impedancematching, Concepts Magn. Res. 5, 57–86 (1993).

. E. Fukushima and S. B. W. Roeder, “Experimental Pulse NMR: ANuts and Bolts Approach,” Addison-Wesley, Reading, MA (1981).

. R. L. Vold, J. S. Waugh, M. P. Klein, and D. E. Phelps, Measurementof spin relaxation in complex systems, J. Chem. Phys. 48, 3831–3832 (1968).

. E. L. Hahn, Spin echoes, Phys. Rev. 80, 580–594 (1950).

. G. C. Borgia, R. J. S. Brown, and P. Fantazzini, Uniform-penaltyinversion of multiexponential decay data, J. Magn. Res. 132, 65–77(1998).

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